The first icon is a movie icon, which represents that the file type is a movie (one that Apple recognizes). The second icon is a generic document icon, because Apple does not recognize MKV files (and AVI on iCloud?) as any particular file type.
It's incorrect (the generic file icon is supposed to be different from a text document icon), but what do you expect from a company that doesn't follow its own HIG and just makes stuff up.
This is what a generic file icon
should be, without any indication that it contains text (because if you don't know the filetype, you don't know if it contains text, and shouldn't report to the user as that):
View attachment 2528881
SF Symbols does have generic document icon, no clue why they aren't using it, yet another inconsistency.
Not file types specifically, but the T&C's don't allow you to upload illegal content, including content you don't have the copyright of.
I wouldn't recommend that.